OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(g) changes in the internal energy of a substance during change of phase;…
Full specification wording
Solid, liquid and gas: (g) changes in the internal energy of a substance during change of phase; constant temperature during change of phase.
During a change of phase, such as melting or boiling, a substance absorbs or releases energy while its temperature stays constant.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 7 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
During a change of phase, such as melting or boiling, a substance absorbs or releases energy while its temperature stays constant. The energy transferred changes the potential energy of the particles as bonds are broken or formed, so the internal energy changes even though the average random kinetic energy, and hence the temperature, does not. For example, melting ice at 0 °C requires energy to break the lattice, but the temperature remains at 0 °C until all the ice has melted. A question may ask what happens to temperature and internal energy during melting or boiling.
What earns the marks
- During a change of phase the temperature of the substance remains constant.
- Energy is transferred to or from the substance during the phase change.
- The internal energy changes because the potential energy of the particles changes.
- The average random kinetic energy of the particles does not change, so temperature stays constant.
- Melting and boiling absorb energy; freezing and condensing release energy.
Common mistakes and how to put them right
Thinking temperature must rise when energy is supplied.
Put it right: During a phase change the temperature stays constant.
Saying internal energy is constant during a phase change.
Put it right: It changes as particle potential energy changes.
Using E = mcΔθ for a phase change.
Put it right: With Δθ = 0 this gives zero, so it does not describe the energy transferred.
Confusing the energy needed to melt a solid with the energy needed to raise its temperature afterwards.
Exam tips
- Identify whether the substance is changing phase or changing temperature before selecting an equation.
- Remember that a horizontal section on a heating graph represents a phase change at constant temperature.
- Link the constant temperature to unchanged average random kinetic energy, and the internal energy change to particle potential energy.
Try the lesson
The start of the Solid, liquid and gas: (g) changes in the internal energy of a substance during change of phase; constant temperature during change of phase. lesson, made from checked teaching notes for this specification point.
Listen
During a change of phase, such as melting or boiling, a substance absorbs or releases energy while its temperature stays constant. The energy transferred changes the potential energy of the particles as bonds are broken or formed, so the internal energy changes even though the average random kinetic energy, and hence the temperature, does not. For example, melting ice at 0 °C requires energy to break the lattice, but the temperature remains at 0 °C until all the ice has melted. A question may ask what happens to temperature and internal energy during melting or boiling. A strong answer earns marks for these points. During a change of phase the temperature of the substance remains constant. Energy is transferred to or from the substance during the phase change. The internal energy changes because the potential energy of the particles changes. The average random kinetic energy of the particles does not change, so temperature stays constant. Exam tip: Identify whether the substance is changing phase or changing temperature before selecting an equation.
See it
Key words
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Change of phase
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Constant temperature during phase change
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Internal energy and particle potential energy
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Energy transfer in melting and boiling
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See every definition
- Change of phase
- A process such as melting or boiling in which a substance absorbs or releases energy while its temperature stays constant.
- Constant temperature during phase change
- The temperature of the substance remains constant because the average random kinetic energy of the particles does not change.
- Internal energy and particle potential energy
- The internal energy changes during a phase change because the potential energy of the particles changes as bonds are broken or formed.
- Energy transfer in melting and boiling
- Melting and boiling absorb energy, while freezing and condensing release energy, transferred to or from the substance during the phase change.
Label the diagram
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